Catalytic cracking is a petroleum refining process that is applied commercially on a very large scale. A majority of the refinery gasoline blending pool in the United States is produced by this process, with almost all being produced using the fluid catalytic cracking (“FCC”) process. In the FCC process, heavy hydrocarbon fractions are converted into lighter products by reactions taking place at high temperature in the presence of a catalyst, with the majority of the conversion or cracking occurring in the gas phase. The FCC feedstock is thereby converted into gasoline, distillate and other liquid cracking products as well as lighter gaseous cracking products of four or fewer carbon atoms. These products, liquid and gas, consist of saturated and unsaturated hydrocarbons.
In FCC processes, feedstock is injected into the riser section of a FCC reactor, where it is cracked into lighter, more valuable products by contacting hot catalyst that has been circulated to the riser-reactor from a catalyst regenerator. As the endothermic cracking reactions take place, heavy carbon is deposited onto the catalyst. This heavy carbon, known as coke, reduces the activity of the catalyst and the catalyst must be regenerated to revive its activity. The catalyst and hydrocarbon vapors are carried up the riser to the disengagement section of the FCC reactor, where they are separated. Subsequently, the catalyst flows into a stripping section, where the hydrocarbon vapors entrained with the catalyst are stripped by steam injection. Following removal of occluded hydrocarbons from the spent cracking catalyst, the stripped catalyst flows through a spent catalyst standpipe and into the catalyst regenerator.
Typically, catalyst is regenerated by introducing air into the regenerator and burning off the coke to restore catalyst activity. These coke combustion reactions are highly exothermic and as a result, heat the catalyst. The hot, reactivated catalyst flows through the regenerated catalyst standpipe back to the riser to complete the catalyst cycle. The coke combustion exhaust gas stream rises to the top of the regenerator through the regenerator flue. The exhaust gas generally contains nitrogen oxides (NOx), sulfur oxides (SOx), carbon monoxide, carbon dioxide, ammonia, nitrogen, and oxygen.
The performance of a fluid catalytic cracking unit can be measured by the conversion of crude hydrocarbon feedstock into useable products such as gasoline. With the addition of a catalytic cracking catalyst, conversion will increase, but so will the production of undesirable side products including coke and hydrogen gas. It is desirable to increase the conversion of an FCC unit while minimizing the increase in coke and H2 byproducts.
The presence of metal contaminants in feedstock presents a serious problem. Common metal contaminants include iron, nickel, sodium, and vanadium. Some of these metals can promote dehydrogenation reactions during the cracking sequence, which can result in increased amounts of coke and light gases at the expense of gasoline production. Metal contaminants can also have a detrimental effect on cracking products. Metal contaminants can deposit on the catalyst and affect its stability and crystallinity. In some cases, the catalyst can be deactivated by the metal contaminants. During the regeneration step, metals present within the catalyst can volatize under the hydrothermal conditions and re-deposit on the catalyst.
For example, vanadium contaminants in feedstock can poison the cracking catalyst and reduce its activity. One theory to explain this poisoning mechanism is that vanadium compounds in the feedstock can become incorporated into the coke deposited on the cracking catalyst, and are then oxidized to vanadium pentoxide in the regenerator as the coke is burned off. Vanadium pentoxide can react with water vapor in the regenerator to form vanadic acid, which can then react with the cracking catalyst to destroy its crystallinity and reduce its activity.
Because compounds containing vanadium and other metal contaminants cannot generally be removed from the FCC unit as volatile compounds, the usual approach has been to passivate these compounds under the conditions encountered during the cracking process. Passivation can involve incorporating additives into the cracking catalyst or adding separate additive particles into the FCC unit along with the cracking catalyst. These additives can preferentially combine with the metal contaminants and act as “traps” or “sinks” so that the active component of the cracking catalyst is protected. Metal contaminants can then be removed along with the catalyst that is withdrawn from the unit during its normal operation. Fresh metal passivating additives can then be added to the unit, along with makeup catalyst, in order to affect a continuous withdrawal of the detrimental metal contaminants during operation of the FCC unit. Depending on the level of metal contaminants in the feedstock, the quantity of additive can be varied relative to the makeup catalyst in order to achieve the desired degree of metals passivation.
Industrial facilities are continuously trying to find new and improved methods to increase the conversion of an FCC unit while minimizing the increase in coke and H2 byproducts. The invention is directed to these and other important ends.